Adsorption Phenomena in Zeolites as Studied by Nuclear Magnetic

4 Adsorption Phenomena in Zeolites as Studied by Nuclear Magnetic Resonance

Downloaded by UNIV LAVAL on July 11, 2016 | http://pubs.acs.org Publication Date: June 1, 1976 | doi: 10.1021/bk-1976-0034.ch004

H. PFEIFER Sektion Physik der Kark-Marx-Universität, DDR 701 Leipzig, Linnéstrasse 5

Introduction Zeolites are porous crystals with a well-defined structure and a high specific surface area, widely used i n industry as molecular sieves, as catalysts, and as ion exchangers. Their common formula is (AlO2)-M+(SiO2)n (1) +

where M denotes an exchangeable cation which can be also replaced by /2 M + or /3 M , and n is a number greater or equal 1. The adsorption phenomena to be discussed i n this report are concerned with molecules i n the pores of a special "family" of zeolites (faujasite group) to which belong the so -called zeolites NaX and NaY generally denoted as NaF. These synthetic zeolites, available as small c r y s t a l l i t e s with a mean diameter between 1 and 100 um, have the same structure and differ only i n the value of n. Their pore system accessible to hydrocarbons, i s a three-dimensional network of nearly spherical cavities, commonly referred to as supercages, each with a mean free diameter of 11.6 Åand connected tetrahedrally through windows of 8 - 9 Ådiameter. Because each s i l i c o n or aluminium ion of the zeolite l a t t i c e i s surrounded by four larger oxygens, the internal surfaces of ideal crystals consist of oxygen, the only other elements exposed to adsorbates being the ex changeable cations which are sodium ions i n the case of inNaFzeolites. Some of these cations are not accessible to Hydrocarbons since they are localized at sites SI and SI' outside of the super cages, some are localized at the walls of the 1

2

1

3+

36

Resing and Wade; Magnetic Resonance in Colloid and Interface Science ACS Symposium Series; American Chemical Society: Washington, DC, 1976.

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Adsorption

Phenomena

in

Zeolites

37

supercages opposite t o the windows ( s i t e s SII) and some c o u l d not be l o c a l i z e d by X-ray measurements, they are assumed, to be l o o s e l y bound ( s i t e s S3) to the w a l l s of the supercages between s i t e s S I I . As was mentioned already i t i s p o s s i b l e t o r e p l a c e Na by other c a t i o n s , and since d i - and e s p e c i a l l y t r i v a l e n t c a t i o n s occupy f i r s t of a l l s i t e s S I and S I , the number of sodium i o n s per supercage can be v a r i e d not only by n, but also through an i o n exchange. Examples are given i n Table I· 1


:

Table 1 ' dumber of exchangeable c a t i o n s i n dehydrated z e o l i t e s of f a u j a s i t e type per Vb u n i t c e l l (corresponding to one supercage; maximum number

Site

2

SI SI' SII

4 4 6

S3

NaX n=1.37

Β aï n=r2.6 0.7 2.3 3.3

1.2 1.7 3.2 4.0

0.4

NaGeY70 ' x

)

η = 2.6

I

1.5 J+1.5

0.5

Nat. 0e£ Ma +

+

(nonloc. ) WceYyO i s a NaY z e o l i t e where 70 % of iMa has been r e p l a c e d by ue^ »

+

+

The p h y s i c a l s t a t e of adsorbed molecules de pends on the r a t i o of i n t e r m o l e c u l a r i n t e r a c t i o n energy to the energy of i n t e r a c t i o n between a mole cule and the s u r f a c e . According to K i s e l e v (1) molecules and surfaces may be c l a s s i f i e d witΕ r e g a r d to t h e i r c a p a c i t y f o r n o n s p e c i f i c and s p e c i f i c i n t e r a c t i o n . Nonspecific i n t e r a c t i o n i s caused mainly by d i s p e r s i o n f o r c e s and i s present t h e r e f o r e i n a l l cases. S p e c i f i c i n t e r a c t i o n may appear a d d i t i o n a l l y whenever e l e c t r o n density (e.g. I f bonds of molecules) or a p o s i t i v e charge (Ma i n the case of Natf z e o l i t e s ) i s l o c a l i z e d on the periphery between the adsorbed molecules and the surface. W have studied the p h y s i c a l state of c y c l i c hydrocarbons, adsorbed i n NaF z e o l i t e s where the degree of s p e c i f i c i n t e r a c t i o n has been s y s t e m a t i c a l l y v a r i e d through the number of ΤΓ e l e c t r o n s per molecule (cyelohexane, cyclohexene, eyelohexadiene benzene and toluene) and through the l o c a t i o n and e

9


38

M A G N E T I C

RESONANCE

number o f sodium i o n s a t t h e w a l l s o f t h e s u p e r cages. Adsorbed

cyclohexane

U s i n g v a r i o u s m i x t u r e s o f c y c l o h e x a n e and d e u t e r a t e d c y c l o h e x a n e i t c o u l d be shown ( 2 ) t h a t t h e magnetic i n t e r a c t i o n w i t h paramagnetic i m p u r i t i e s of the z e o l i t e ( c a l l e d proton e l e c t r o n interaction) i s c o n t r o l l i n g the proton r e l a x a t i o n t i m e s (1L and T~) of C E L a d s o r b e d i n a c o m m e r c i a l NaY z e o l i t e (VEB Chemlekumbinat i i i t t e r f e l d , η =2.6) These, p a r a m a g n e t i c i m p u r i t i e s a r e F e ^ i o n s l o c a t e d at A l ^ * s i t e s o f t h e z e o l i t e s k e l e t o n , a l t h o u g h an even g r e a t e r number i s l o c a l i z e d a t s i t e s S I o r a s an i r o n o x i d e - l i k e phase on t h e c r y s t a l s ( 3 ) · The t o t a l c o n c e n t r a t i o n , d e t e r m i n e d o p t i c a l l y c o r r e sponds t o about ^QO ppm Fe^O^ f o r t h i s z e o l i t e . The t e m p e r a t u r e dependence ox I L and T h a s been measured a t 16 MEz between - 140°C and +90°C f o r p o r e f i l l i n g f a c t o r s θ = 0.16, 0.4, 0 . 7 , and 0.88. T h i s q u a n t i t y θ i s t h e r a t i o o f t h e a d s o r b e d amount of c y c l o h e x a n e t o i t s maximum v a l u e w h i c h c o r r e sponds t o a o o u t f o u r m o l e c u l e s p e r s u p e r c a g e . I n a l l c a s e s t h e minimum o f Τ.·, o c c u r e d a t t h e same t e m p e r a t u r e (-100°U) w h i l e t h e r a t i o o f l V | t o 1*2 a t "uhis p o i n t i n c r e a s e d f r o m a b o u t 2 t o 4, and T^ v e r s u s r e c i p r o c a l a b s o l u t e t e m p e r a t u r e becomes f l a t t e r w i t h i n c r e a s i n g coverage. Using T o r r e y s model f o r t r a n s l a t i o n a l m o t i o n ( 4 ) t h e f o l l o w i n g f o r m u l a e c a n be d e r i v e d f o r t h i s c a s e (£) where £ a n d CJ^ a r e t h e Larmor f r e q u e n c i e s o f t h e p r o t o n and p a r a m a g n e t i c i o n r e s p e c t i v e l y , Κ i s a constant proportional to the concentration o f p a r a m a g n e t i c i o n s , d d e n o t e s t h e minimum d i s t a n c e between p r o t o n and i o n ^ a n d C i s t h e c o r r e l a t i o n time f o r t h e t r a n s l a t i o n a l motion o f t h e adsorbed molecule: c


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Adsorption Phenomena in Zeolites

39

where OC

-

(5)

» 2 The mean q u a d r a t i c jump l e n g t h < r > , t h e a v e r a g e t i m e V between two jumps a n d t h e d i f f u s i o n c o n s t a n t D a r e r e l a t e d by E i n s t e i n ' s e q u a t i o n Downloaded by UNIV LAVAL on July 11, 2016 | http://pubs.acs.org Publication Date: June 1, 1976 | doi: 10.1021/bk-1976-0034.ch004

The f u n c t i o n f

t

= ( oc ,

6 3>Γ.

(6)

V ) i s defined as c

(7)

where 01

A f i t t i n g p r o c e d u r e ( 6 ) y i e l d s OC ^1/5 f o r θ = 0 . 8 8 , and s i n c e f o r t h i s v a l u e o f at j a t t h e minimum of T i t i s UJ^X^s 1.58, we h a v e 1

I n c o n t r a s t , f o r t h e l o w e s t c o v e r a g e (θ = 0·16) i t f o l l o w s from the f i t t i n g procedure B e c a u s e i n t h i s c a s e t h e minimum o f occurs i f Od^V = 1§ so t h a t we have c

T h e r e f o r e "C i s a b o u t 1 6 .·· 2 0 n s a t ~100°C a n d does n o t depend on c o v e r a g e w i t h i n t h e s e l i m i t s . The mean q u a d r a t i c jump l e n g t h o f t r a n s l a t i o n a l m o t i o n however, d e c r e a s e s w i t h i n c r e a s i n g c o v e r a g e which i s a d i r e c t h i n t f o r n o n l o c a l i z e d adsorption o f c y e l o h e x a n e . I n a c c o r d a n c e w i t h t h i s , measure ments o f p r o t o n r e l a x a t i o n i n z e o l i t e s w i t h différait


40

MAGNETIC

RESONANCE


number o f s o d i u m i o n s ( n = 1.37.(2) a n d n = 1 * 2 , ( 8 ) ) r e v e a l e d a b o u t t h e same c o r r e l a t i o n t i m e s as i n t h e above s y s t e m ( n = 2.6)· S i n c e t h e s e z e o l i t e s a r e o f h i g h p u r i t y ( o n l y 2 . . . 3 PPm JtepCU i n t h e c a s e o f η = 1.37) a n d t h e c r y s t a l l i t e s o f H s x g d i a m e t e r ( 1 5 ... 2 5 / u m i n t h e c a s e o f η = 1 . 2 ) i t was p o s s i b l e t o d e t e r m i n e t h e i n t r a c r y s t a l l i n e s e l f - d i f f u s i o n c o e f f i c i e n t D by t h e p u l s e d f i e x d g r a d i e n t t e c h n i q u e , The r e s u l t s a r e shown i n Table 2 . Table 2 S e l f - d i f f u s i o n c o e f f i c i e n t o f C^-H^p i n E a X z e o l i t e s a t room t e m p e r a t u r e ( D ^ ) and a c t i v a t i o n e n e r g y ( Ε ) 2

η = 1.37 θ = 0.3

20 = 7

β

10~

fo

2

cm /s

E

D

= ( 3 . 0

±

0 . 2 ) g g "

(2) η = θ

1*2 0.25 2

"20

[cm /s]

5.5*10'

0.2?

0.45

0.66

-6 6.10' -6 3.8.10-6 9*10'r 7

υ. 69 ^ 7 5 9.10

(â) From equs. ( 1 0 ) and ( 1 1 ) and t h e o b s e r v e d a c t i v a t i o n energy ή = ( 3 · 0 + 0 . 3 ) k c a l / m o l e , t h e v a l u e o f V a t room t e m p e r a t u r e c o m e s o u t ijo be ~ 0 . 5 n s s o t h a t t h e rms jump l e n g t h | ( c f . equ.(6)) and T a b l e 2)isabout 1 3 S , a n d l e s s t h a n 2 X t o r θ = υ · 2 and 0 · 8 r e s p e c t i v e l y . Summarized, c y c l o h e x a n e a d s o r b e d i n z e o l i t e s o f f a u j a s i t e t y p e behaves l i k e an i n t r a c r y s t a l l i n e l i q u i d : t h e m o l e c u l e s do n o t jump b e t w e e n s i t e s w h i c h a r e f i x e d i n s p a c e . The mean jump l e n g t h d u r i n g t r a n s l a t i o n a l m o t i o n d e c r e a s e s w i t h i n c r e a s i n g coverage as a consequence of mutual h i n d r a n c e , w h i l e t h e average t i m e between jumps does n o t change s i g n i f i c a n t l y . 9

Adsorbed n o n s a t u r a t e d c y c l i c hydrocarbons In Table 3 the r a t i o s o f proton r e l a x a t i o n times a n d Tp measured a t t h e minima o f for v a r i o u s c y c l i c h y d r o c a r b o n s a d s o r b e d i n t h e same c o m m e r c i a l NaY z e o l i t e (n=2.6) a s above, a r e l i s t e d

(2) Resing and Wade; Magnetic Resonance in Colloid and Interface Science ACS Symposium Series; American Chemical Society: Washington, DC, 1976.

4.

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41

Table 3 T^/Tp a t t h e minimum o f TA$ a c t i v a t i o n e n e r g y 1 and c o r r e l a t i o n t i m e T 'for various c y c l i c h y d r o c a r b o n s a d s o r b e d i n a c o m m e r c i a l NaT z e o l i t e ( n s 2.6) c

Η


θ =0.2 θ =0.8 c

LiôIëJ

at

P

20 X5

°6 6

°6 8

Η

°6 10

°6 12

1.8

1.8

1.8

2.1

1.8

1.9

1.9

Θ

=0.2

Θ

=0.8

e-

=0.2 13

Θ

=0.8

|

50

3.4

±

Η

0.3

3

7.7 '

Η

2.8

8

9

+ 0.3

r =^2~0.25 t

V=T

~ 0 . 5

As c a n b e s e e n , t h e s e r a t i o s a r e c l o s e t o t h e t h e o r e t i c a l v a l u e (1·83) f o r d o m i n a t i n g p r o t o n e l e c t r o n i n t e r a c t i o n and t h e r m a l m o t i o n c h a r a c t e r i z e d by a s i n g l e c o r r e l a t i o n time V (10). T h e r e f o r e we h a v e c

and C c a n be r e a d i l y d e t e r m i n e d a s a f u n c t i o n o f T, s i n c e T,. h a s b e e n measured between about -Ί00 U a n d + 2 0 0 ° G ( ^ ) . The l o g a r i t h m o f T p l o t t e d v e r s u s Ύ Τ g i v e s s t r a i g h t l i n e s , so t h a t an a c t i v a t i o n e n e r g y Ε c a n be d e f i n e d w h i c h i s l i s t e d i n T a b l e 2 together with absolute values f o r a t room t e m p e r a t u r e , u'rom t h e s e r e s u l t s t h e f o l l o w i n g c o n c l u s i o n s concerning non^jsaturated c y c l i c hydrocarbons a d s o r b e d i n NaX c a n be drawn: ( i ) E v e n a t h i g h p o r e f i l l i n g f a c t o r s (Θ=0.8) t h e r e i s no d i s t r i b u t i o n o f c o r r e l a t i o n t i m e s : The m o l e c u l e s a r e a d s o r b e d a t s i t e s f i x e d i n s p a c e ( l o c a l i z e d a d s o r p t i o n ) . These a d s o r p t i o n c e n t r e s w i l l be t h e sodium i o n s a t s i t e s S I I as we s h a l l s e e l a t e r . ( i i ) As a c o n s e q u e n c e o f l o c a l i z e d a d s o r p t i o n u j y r i n g ^ e q u a t i o n c a n be a p p l i e d : c

1

c


42

MAGNETIC


v

c =

ép

4-

z*?t t/nT)

A

RESONANCE

( 1 3 )

B

where t h e i n i t i a l s t a t e ( p a r t i t i o n functioç ) c o r r e s p o n d s t o t h e m o l e c u l e a d s o r b e d o n l a and t h e a c t i v a t e d s t a t e ( p a r t i t i o n f u n c t i o n ρ^φ) c o r r e s p o n d s t o t h e m o l e c u l e d u r i n g i t s jump f r o m one Ν a t o a n o t h e r . I n a c c o r d a n c e w i t h t h i s m o d e l the energy o f a c t i v a t i o n Ε i s l e s s t h a n t h e heat o f a d s o r p t i o n ( & = 1 8 Jfccal/mole f o r b e n z e n e ) . I t does n o t depend o n t h e number o f ΤΓ e l e c t r o n s . The o b served increase o f V from G H t o 0 Η must be e x p l a i n e d by a change o f t h e p r e e x p o n e n t i a l f a c t o r and e s p e c i a l l y b y a d e c r e a s e o f since a stronger i n t e r a c t i o n always l e a d s t o a decrease i n t h e p a r t i t i o n f u n c t i o n . C o r r e s p o n d i n g l y , we have a n increasing r e s t r i c t i o n o f molecular mobility i n t h e a c t i v a t e d s t a t e w i t h i n c r e a s i n g number o f 1Γ e l e c t r o n s . ( i i i ) The i n c r e a s e o f V w i t h i n c r e a s i n g c o v e r a g e i s a l s o i n accordance w i t h e q u . ( 1 3 ) because t h e p r e e x p o n e n t i a l f a c t o r i s p r o p o r t i o n a l t o /u, where u d e n o t e s t h e number o f u n o c c u p i e d a d s o r p t i o n s i t e s w h i c h c a n be r e a c h e d f r o m t h e a c t i v a t e d s t a t e : We c a n assume u ^ 3 and u £ 1 f o r Θ» = 0.2 a n d 0.8 r e s p e c t i v e l y , s i n c e t h e r e a r e about 4 a d s o r p t i o n c e n t r e s p e r s u p e r c a g e , and θ = 1 c o r r e s p o n d s r o u g h l y t o t h e same number o f m o l e c u l e s . F o r a f u r t h e r c h e c k o f t h i s model we have measured the p r o t o n r e l a x a t i o n times o f t o l u e n e adsorbed on t h e same z e o l i t e . A l t h o u g h benzene a n d t o l u e n e d i f f e r i n t h e i r i n t e r m o l e c u l a r i n t e r a c t i o n s , as c a n be s e e n f r o m t h e i r q u i t e d i f f e r e n t m e l t i n g p o i n t s (+ 5 · 5 ° 0 a n d -95°C r e s p e c t i v e l y ) , m o l e c u l a r m o t i o n i n t h e adsorbed s t a t e s h o u l d be s i m u l a r , s i n c e i t i s d e t e r m i n e d b y t h e i n t e r a c t i o n between t h e r i n g o f t h e m o l e c u l e s and t h e sodium i o n s a t s i t e s S I I . T h i s i n d e e d h a s b e e n o b s e r v e d : There i s no d i f f e r e n c e i n p r o t o n r e l a x a t i o n t i m e s between benzene a n d t o l u e n e f r o m a b o u t -20°0 t o +180°C ( 1 1 ) . I n a d d i t i o n , t h i s r e s u l t states that the r o t a t i o n of t h e benzene molecule around i t s s i x f o l d a x i s o f symmetry does n o t i n f l u e n c e p r o t o n r e l a x a t i o n i n t h i s temperature i n t e r v a l . 0

6

1 0

&

6

c

1


4.

PFEiFER


43

I f benzene i s a d s o r b e d i n a c o m m e r c i a l ΙΗΘΧ z e o l i t e (VEB Ohemiekombinat B i t t e r f e l d , η = Ί . 8 , A^500 ppm F e 0 o ) t h e minimum o f s h i f t s t o much l o w e r t e m p e r a t u r e s ( f i g . 1) a n d t h e r a t i o Ty./Tp i n c r e a s e s s l i g h t l y f r o m 1 . 8 t o about 2 . 4 · T h e r e f o r e t h e m o b i l i t y o f benzene m o l e c u l e s a d s o r b e d i n NaX i s much h i g h e r ( a t room t e m p e r a t u r e V * Λ 0 n s i n c o m p a r i s o n t o 50 n s i n NaX) a n d a s l i g h t d i s t r i b u t i o n o f c o r r e l a t i o n t i m e s seems p r o b a b l e . The much h i g h e r m o b i l i t y o f benzene m o l e c u l e s a d s o r b e d i n HaZ z e o l i t e s h o u l d b e d i s c u s s e d i n c o n n e c t i o n w i t h r e s u l t s f o r w a t e r where t h e m o b i l i t y d o e s n o t depend o n s i l i c o n t o a l u m i n i u m r a t i o η o f z e o l i t e s (12). F o r a pore f i l l i n g f a c t o r θ s 0 . 8 t h e r e a r e about 24 w a t e r m o l e c u l e s and o n l y about 4 b e n z e n e m o l e c u l e s p e r s u p e r c a g e . T h e r e f o r e i n NaX ( n = 2 . 6 ) a l l a r o m a t i c m o l e c u l e s w i l l b e s t r o n g l y bound b y s p e c i f i c i n t e r a c t i o n to t h e l o c a l i z e d sodium i o n s which a r e f i x e d a t the w a l l o f t h e supercage ( s i t e s S I I , c f . T b l e 1 ) · This e x p l a i n s t h e low m o b i l i t y o£ t h e s e m o l e c u l e s . I n NaX z e o l i t e s ( n = 1 . 8 ) however, a r o m a t i c m o l e c u l e s w i l l be bound t o t h e n o n l o c a l i z a b l e s o d i u m i o n s a v a i l a b l e h e r e , s i n c e t h e s e a r e more e x p o s e d than N a a t s i t e s S I I and a c t as primary a d s o r p t i o n c e n t r e s ( 1 3 ) » So t h e h i g h m o b i l i t y i n NaX c a n be e x p l a i n e d T5y t r a n s l a t i o n a l a n d r o t a t i o n a l m o t i o n o f a r o m a t i c m o l e c u l e - s o d i u m ion-complexes· I n agreement w i t h t h i s , i t was f o u n d r e c e n t l y (14) t h a t w i t h i n c r e a s i n g S i / A l - r a t i o (n) t h e m o b i l i t y decreases a t f i r s t , b u t r e m a i n s c o n s t a n t above η = 2 . 4 3 where a l l s o d i u m i o n s a r e l o c a l i z e d , a n o t h e r model h a s been proposed b e f o r e (15), a c c o r d i n g t o w h i c h t h e d i f f e r e n c e i n m o b i l i t y i s caused by d i f f e r e n t e l e c t r i c f i e l d s i n NaX a n d NaX z e o l i t e s . B u t t h i s m o d e l w o u l d p r e d i c t a monotonous d e c r e a s e o f mobility with increasing values of η oppositet o the e x p e r i m e n t a l r e s u l t s . I n c o n t r a s t t o benzene, f o r w a t e r a t h i g h e r p o r e f i l l i n g f a c t o r s (Θ 0 . 5 ) the mobility o f a d s o r b e d m o l e c u l e s does n o t depend on n , s o that the reduced m o b i l i t y w i t h respect t o l i q u i d w a t e r c a n n o t be c a u s e d b y t h e sodium i o n s . T h i s i s c o n f i r m e d by two e x p e r i m e n t a l f i n d i n g s : F i r s t l y , e v e n i n c o n c e n t r a t e d aqueous s o l u t i o n s o f N a t h e m o b i l i t y o f water m o l e c u l e s i s only s l i g h t l y r e duced a n d c o r r e s p o n d s a t room t e m p e r a t u r e t o a n increase o f c o r r e l a t i o n time from the f r e e water v a l u e 2 . 5 p s (16) t o o n l y about 5 p s (17) f o r a 2


c

a

+

+


44

M A G N E T I C

RESONANCE

water molecule i n the f i r s t c o o r d i n a t i o n sphere o f a sodium i o n . S e c o n d l y t h e s m a l l w i d t h o f t h e d i s t r i b u t i o n f u n c t i o n f o r V (11), (12) i s e q u i v a l e n t to the statement t h a t the c o r r e l a t i o n time of a w a t e r m o l e c u l e a d j a c e n t t o a sodium i o n c a n n o t be v e r y d i f f e r e n t from the c o r r e l a t i o n time o f water m o l e c u l e s s u r r o u n d e d by oxygen atoms b e l o n g i n g t o the w a l l o f t h e s u p e r c a g e o r t o o t h e r w a t e r m o l e c u l e s , because the e x p e r i m e n t a l l y determined h a l f width of the d i s t r i b u t i o n function ( f o r the distri b u t i o n p a r a m e t e r β a s d e f i n e d i n (18) i t has been found B é 1 ) corresponds t o a r a t i o o f c o r r e l a t i o n t i m e s o f l e s s t h a n about 5 · f h i s o n l y s m a l l change i n m o b i l i t y o f w a t e r m o l e c u l e s c a u s e d by N a i s due t o t h e s t r o n g w a t e r - w a t e r i n t e r a c t i o n (19) and t h e r e f o r e t h e r e d u c e d m o b i l i t y i n t h e a l s o r b e d s t a t e ( Τ \ =200 ps at room t e m p e r a t u r e ) must be a c o l l e c t i v e e f f e c t o f the r i g i d oxygen l a t t i c e o f t h e z e o l i t e w h i c h s t a b i l i z e s t h e ensemble o f w a t e r m o l e c u l e s i n front of i t (,"stabilization e f f e c t " (11)). To show d i r e c t l y t h e i n f l u e n c e o f s o d i u m i o n s on t h e m o b i l i t y o f Denzene, measurements o f p r o t o n r e l a x a t i o n t i m e s i n NaCeY 70 z e o l i t e have b e e n . p e r f o r m e d , where t h r o u g h an exchange o f 70 % Na by C e ^ t h e number o f sodium i o n s i n t h e s u p e r c a g e s has been d r a s t i c a l l y r e d u c e d ( c f . T a b l e 1 ) . A l t h o u g h t h e minimum o f T^ i s s h i f t e d t o extreme low t e m p e r a t u r e s ( t o l e s s t h a n -1^0°ϋ (19)) t h i s c a n n o t be t a k e n a s a p r o o f f o r a h i g h e r m o b i l i t y o f t h e a d s o r b e d benzene m o l e c u l e s , s i n c e i t i s an a r t i f a c t o f n u c l e a r m a g n e t i c r e l a x a t i o n method. The v a l u e s o f t h e r e l a x a t i o n t i m e s a r e significantly g r e a t e r t h a n i n NaY i n c o n t r a s t t o the u s u a l e f f e c t i f paramagnetic i o n s (Qe^*) a r e i n t r o d u c e d i n t o a sample. The r e a s o n i s t h a t Ce^ r e d u c e s by m a g n e t i c d i p o l e i n t e r a c t i o n the e l e c t r o n r e l a x a t i o n t i m e o f t h e p a r a m a g n e t i c i m p u r i t i e s which c o n t r o l (Pe3 ions located at A l ^ s i t e s of the z e o l i t e s k e l e t o n ) the proton r e l a x a t i o n o f a d s o r b e d benzene t o s u c h a v a l u e t h a t i s becomes shorter than the thermal c o r r e l a t i o n time (19). T h e r e f o r e a z e o l i t e was p r e p a r e d where t h e very s i m i l a r but d i a magnetic L a ^ , i n s t e a d of the p a r a m a g n e t i c (Je3 was i n t r o d u c e d . The r e s u l t , shown i n P i g . 1, i s i n f u i l agreement w i t h t h e above model, s i n c e one o b s e r v e s t h a t t ? a t room t e m p e r a t u r e d e c r e a s e s f r o m 50 n s f o r NaY t o 10 ns or t o ΝαΥ, η = 2.6; ·, ΝαΧ, η = 1.8; Χ, NaLaY74,n = 2.6



46

MAGNETIC

RESONANCE

introduced (NaX) or i f these and the l o c a l i z e d sodium ions are removed from the supercageâ (NaLaY 74). The broad d i s t r i b u t i o n of c o r r e l a t i o n times i n the l a t t e r case which can be deduced from the great value of T^/Tp at the minimum of T* may be due t o a few t i g h t l y bound benzene molecules (e.g. at the few ha+ s t i l l occupying s i t e s S I I ) and p a r t l y due t o n o n l o c a l i z e d adsorption o f the major part of adsorbate (small jump lengths during the course o f t r a n s l a t i o n a l motion). F i n a l l y i t shoula be mentioned that the benzene-sodium ion-complex suggested by these measurements was studied also t h e o r e t i c a l l y using the semiemyirical quantum chemical Ci\D0/2 method (20) . From energy minima of f u l l p o t e n t i a l curves tïïe optimum arrangement and s t a b i l i z a t i o n energy coula oe obtained. The π est favourable c o n r i g u r a t i o n has been ιουηα when the sodium i o n i s l y i n g on the s i x f o l d axis o f symmetry of the aromatic r i n g (3·2 8 j , because i n t h i s case there i s a good p o s s i b i l i t y f o r overlapping of the tree e l e c t r o n o r b i t a l s of N a with the if o r b i t a l s of the adsorbed molecules, ana an e l e c t r o n t r a n s f e r w.s found from the If system towara the c a t i o n . This mouel of benzene adsorption nas "been confirmed experimentally (21)fcnrar broad l i n e technique, i n s o f a r , as the independence o f second moments of proton resonance at 77 Κ on pore f i l l i n g f a c t o r s could be explained only by a f l a t arrangement o f benzene molecules i n f r o n t o f sodium ions occupying s i t e s S I I . +

c

References (1) (2) (3) (4) (5) (6) (7) (8) (9)

Kiselev, A . V . , Disc.Farad.Soc. (1966) 40, 205-218 Michel, D . , Thöring, J., Z.phys.Chemie (Leipzig) (1971) 247, 85-90 Derouane, E . G . , Mestdagh, M.M., Vielvoye, L., J.Catalysis (1974), 33, 169-175 Torrey, H . C . , Phys.Rev. (1953) 92, 962-969 Krüger, G.J., Z.Naturforsch. (1969), 24a, 560-565 Michel, D . , Thesis (Prom.B) Karl-Marx-Uni versität Leipzig (1973) Lorenz, P . , Diplomarbeit, Karl-Marx-Uni versität Leipzig (1974) Walter, Α . , Thesis i n prep. Karl-Marx-Uni versität Leipzig Nagel, Μ., Pfeifer, Η . , W i n k l e r , Η . , Ζ.phys.Chemie (Leipzig) (1974), 255, 283-292


4. PFEIFER



(10)

47

Pfeifer, Η . , NMR - Basic Principles and Progress Vol.7, p.53-153, Springer, New York 1972 (11) Pfeifer, Η . , Surface Phenomena Investigated by Nuclear Magnetic Resonance, Phys.Reports (in press) (12) Pfeifer, Η . , Gutsze, Α . , Shdanov, S . P . , Z.phys.Chemie (Leipzig) (in press) (13) Dzhigit, O.M., Kiselev, A.V. et al., Trans.Farad.Soc.(1971), 67, 458-467 (14) Lechert, H . , Haupt, W., Kacirek, H., Ζ . N a t u r f o r s c h . (1975), 30a, 1207-1210 (15) L e c h e r t , Η . , Hennig, H.J., Mirtsch, Sch., Surface S c i . (1974), 43, 88-100 (16) Hindman, J.C., Z i e l e n , A.J. et al., J.Chem.Phys. (1971), 54, 621-634 (17) Hertz, H . G . , Angew.Chemie (1970), 82, 91-106 (18) Nowick, A . S . , Berry, B . S . , JBM J.Res. Development (1961), 5, 297-320 (19) Geschke, D., Pfeifer, Η . , Z.phys.Chemie (Leipzig) (in press) (20) Geschke,D., Hoffmann, W.-D., Deininger, D . , Surface S c i . (in press) (21) Hoffmann, W.-D., Z.phys.Chemie (Leipzig) (in press)


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